The invention concerns a multiple leaf collimator for limiting a bundle of high-energy rays emitted by a substantially point-like radiation source and directed towards a treatment object and used in particular for the stereotactic conformation radiotherapy of tumors, wherein the collimator contains a plurality of opposing collimator leaves made of a radiation-absorbing material which can be moved by drive mechanisms into the optical path such that the contours of said optical path can be freely defined, wherein the front edges of the collimator leaves are always aligned in parallel to the optical path.
The treatment devices used today in oncological radiation therapy are provided with collimators which limit high-energy radiation, in most cases high-energy gamma radiation from a linear accelerator, in such a fashion that the rays assume exactly the same shape as the object to be treated. Since irradiation e.g. of a tumor, is implemented from different directions, a high irradiation intensity on the tumor can be effected with only limited exposure to the surrounding tissue. For absorbing high-energy radiation, the collimator must have a thickness of several centimeters, which produces a half shadow when the passage opening has straight walls in the passage direction. Since the rays diverge from the substantially point-like radiation source, the collimator opening is smaller than the actual shape of the tumor so that the collimated rays diverge to have exactly the size of the tumor upon impingement. When the walls of the collimator opening are straight, part of the radiation will not be shielded by the full material thickness due to the inclined path of the radiation. In consequence thereof, either healthy tissue surrounding the tumor is exposed to considerable radiation or the tumor tissue will receive too little dosage. This causes damage which should be prevented. For this reason, one of average skill in the art has tried to develop different collimators which reduce or prevent these half shadows.
One suggestion to prevent half shadows which has been described in the literature, consists in providing the collimator leaves (leaves) of a collimator (multi-leaf collimator) with an irregular trapezoidal shape such that their side surfaces and the side surfaces of the outer limits of the collimator opening have the angle of the optical path. It is, however, more difficult to achieve corresponding alignment of the front edges of the collimator leaves. Many suggestions have been made to solve this problem, none of which is satisfying.
In one suggestion made e.g. in EP 0 259 989 B1, EP 0 556 874 B1, EP 0 562 644 B1, U.S. Pat. No. 5,166,531 and DE 33 11 870 C2, the front edges of the collimator leaves have a rounded shape such that the outer rays of the bundle contact these front edges tangentially. Through this solution, the half shadow can be weakened but not prevented. The same is true for a further suggestion made in EP 0 259 989 B1, EP 0 556 874 B1 and EP 0 562 644 B1, wherein the radiation must pass two sequential collimator openings. In DE 195 04 054 A1, such graduation of the front edges of the collimator leaves was further refined by constructing each collimator leaf from a plurality of rods, disposed one on top of the other such that they can be displaced with respect to one another. This collimator is complicated due to the large number of parts and exhibits increased radiation leakage due to the bordering of many collimator leaf elements and the associated unavoidable tolerances. Moreover, no drive mechanism is provided. Adjustments must be made by hand and automatic computer-controlled adjustment of the collimator opening is not possible.
DE 33 11 870 C2, U.S. Pat. No. 3,151,245, U.S. Pat. No. 4,987,309, U.S. Pat. No. 5,144,647, EP 0 193 509, EP 0 245 768 B1, the substantially idendical EP 0 387 921, and EP 0 314 214 B1 proposed moving the collimator leaves along curved paths such that the front edges of the collimator leaves are always aligned parallel to the optical path. This requires complicated guidance of the collimator leaves. The arrangement of such complicated guidance means imposes limits on the goal of minimizing the thickness of the collimator leaves. Collimator leaves must be thin to exactly reproduce the shape of the tumor, since rough graduations result in healthy tissue also being irradiated and destroyed or badly damaged. Moreover, if the collimator leaves have the shape of irregular trapezoids and are guided on curved paths, jamming can occur in consequence of this geometrical shape. To prevent same, DE 37 11 245 A1 proposes tapering the collimator leaves towards their front end facing the optical path. Wide opening of a collimator of this type produces gaps which cause increased leakage of rays. Finally, the problems of EP 0 314 213 B1 and U.S. Pat. No. 4,987,309 were believed to be solved by disposing the trapezoidal collimator leaves and the collimator leaves which can be displaced on curved paths such that the bundle of rays must pass through both collimator openings. Although each of the two collimators has a half shadow which is reduced by the other respective collimator, half shadows can only be eliminated with twice the shielding, i.e. almost twice as much material thickness is required. The amount of effort needed for drive and control is also doubled.
In addition, collimators have been disclosed (FR A 2519 465 and EP A 0286 858) which are composed of two pairs of shielding blocks offset from each other by 90xc2x0. These shielding blocks have front and rear components for preventing half shadows, wherein the latter can be aligned parallel to the optical path. The front blocks have sidewardly projecting bearing pins for pivoting on a holding device which is also connected to the rear blocks and on which drive means act for adjustment. These collimators can, however, only define a rectangular beam and a shape in the form of an object which is to be treated within a living organism, such as a tumor, can not be generated. This requires a shaping multiple leaf collimator of the above mentioned kind. The technical solution cannot easily be transferred to a multiple leaf collimator since a sideward bearing of forward components of this type cannot be effected with leaves of a multiple leaf configuration at those locations at which an adjacent leaf must be disposed for forming the above mentioned shape. A strict requirement for multiple leaf collimators is the absence of any shielding gaps between the leaves forming the shape since the associated leakage would destroy healthy tissue.
It is therefore the underlying purpose of the invention to solve the above-mentioned problems and to produce a multiple leaf collimator which eliminates half shadows with as little effort as possible.
This object is achieved in accordance with the invention in that the collimator leaves consist of a rear part which can be displaced linearly, and a front part connected thereto, wherein the front part of each collimating leaf is adjusted in correspondence with the respective position of the associated rear part through drive means such that the front edges are always aligned parallel to the optical path and such that the connection between the front part and the rear part does not lead to any significant gaps in the volume of the radiation absorbing material.
The present invention omits complicated curved displacement of the collimator leaves to simplify mechanics and reduce leaking radiation, since the linear displacement permits closer tolerances. From the point of view of mechanics and drive technology, adjustment of the front part can be realized in a considerably better and easier fashion compared to the curved displacement of prior art. The complete surface of the full material thickness is used for shielding to completely eliminate half shadows without requiring either the increased effort or additional shielding of the above-mentioned prior art. The suggested technical solution is superior with respect to the previous approaches, in particular when the collimator leaves have a trapezoidal shape to also prevent the half shadow caused by the side surfaces of the collimator leaves. The linear guidance avoids jamming even when the collimator leaves are trapezoidal which permits closer tolerances and reduction of leakage compared to collimator leaves with curved guidance. Only the front parts require a somewhat increased tolerance to prevent interference during adjustment in consequence of the trapezoidal shape. This tolerance is minimal compared to that of a curved guidance system. It is clear that the invention is not limited to trapezoidal collimator leaves in dependence on the size of the collimator and the angle of the optical path.
The invention provides that the front part is pivoted on the rear part, such that the volume of the ray-absorbing material is substantially uninterrupted. This should be taken into consideration in the concrete embodiment of pivoting, wherein there are several possibilities which will be explained below.
It is possible to provide separate drives for displacing the rear parts of the collimator leaves and for adjustment of the front parts, respectively, wherein these are matched by computer control. The drive means are preferably designed such that forced mechanical coupling into each position of the rear part guarantees the associated alignment of the front part and thereby the front edges, to prevent misalignment of the front edges in consequence of program or drive means error. The reliability is considerably increased, which is particularly important for patients and users. Further advantages of this embodiment consist in that only one drive is required for each collimator leaf which requires correspondingly less computer work to thereby obtain more rapid calculation results and faster adjustment of the collimator to another shape.
The front part can be coupled to the rear part in many different ways. The end of the rear part can e.g. have a rounded shape and a front part with a corresponding rounded shape can be disposed thereon. It is also possible to combine segment-shaped front parts with corresponding recesses in the rear parts. However, the front parts are preferably substantially semi-circular bodies which are securely borne in corresponding recesses at the front end of the rear parts, wherein adjustment comprises a pivoting motion about the imaginary axis of rotation at the center of the circular shape. There are different possibilities of secure mounting without considerably interrupting the volume of the radiation-absorbing material, e.g. dove-tailed guidance means, guidance in grooves, retaining pins guided in slots, etc. The height of the rear part preferably substantially corresponds to the diameter of the semi-circular body, wherein the front ends of the rear part are displaced to the rear such that any required inclined position of the front edges of the collimator leaves is possible. This embodiment has the advantage that the pivoted front part also has the same height as the associated rear part for all possible positions. This is advantageous for the concrete design for bearing and guiding the collimator leaves.
The cross-sections of the collimator leaves preferably have asymmetrical trapezoidal shapes such that their side surfaces extend approximately parallel to the optical path, wherein the inner surfaces of the lateral sides bordering the outer collimator leaves extend at an inclination such that they join with the outer collimator leaves without leaving gaps. In this fashion, a half shadow is prevented since all limitations of the collimator opening correspond approximately to the optical path. As was mentioned above, such an embodiment of the collimator in accordance with the invention is very advantageous. The front parts preferably have sufficient lateral play to guarantee adjustment, despite the trapezoidal shape.
One embodiment provides that the collimator leaves can be displaced beyond the central line of the possible collimator opening to permit reproduction of tumors having any irregular contour, e.g. including U-shaped contours which require that the collimator leaves cross the central line of the collimator opening. This embodiment also facilitates modulation of the intensity of the rays through temporary covering of certain regions. A further advantage is that the collimator leaves can be closed asymmetrically, e.g. like a zipper. This considerably reduces leakage of rays in the closing region compared to that associated with closing all collimator leaves in the center. Clearly, to achieve such displacement beyond the central line, the length and distance of displacement of the collimator leaves must be dimensioned correspondingly.
In order to position the collimator leaves (like in the above-mentioned EP 0 245 768 B1 and in the largely identical EP 0 387 921 A1) a drive mechanism may be provided which adjusts several collimator leaves, one after the other. It is, however, preferred to provide each collimator leaf with one single controllable drive to permit quick computer-controlled shape changes. This is particularly important for dynamic irradiation of a tumor, wherein irradiation is enabled from different sides with relatively frequent changes. Even if the object of irradiation has an irregular shape and rapid change of the contour is required, maximum protection of the surrounding tissue is thereby ensured. The individual drives are also suitable if collimator leaves must be temporarily moved into the collimator opening during irradiation to weaken the intensity of radiation in certain regions. To increase the safety and reduce the number of drives, these individual drives preferably exhibit the forced coupling mentioned above.
Control of the collimator during operation thereof is preferably effected by a computer which adjusts the contour and position of the collimator opening to the object of irradiation in the respective direction of radiation, wherein the computer receives the data from a device for detecting the shape of the object of radiation and a control means examines the result of the adjustment. The collimator leaves can thereby be advantageously disposed in a displaceable collimator block which is provided for positioning the collimator opening relative to the object of irradiation and to the radiation source. The collimator block can be divided along a central line which permits separate displacement of these halves. Moving apart of the two halves additionally increases the collimator opening. The collimator block can also be mounted to a gantry which permits a relative motion between the collimator and the patient such that the patient can be irradiated from all sides through adjustment of the collimator opening to the shape of the object of irradiation. In this fashion, the collimator can be used to encircle a tumor to be irradiated, wherein this motion must not necessarily be circular but can also extend through three dimensions. A radiation method of this type is known, but is facilitated in combination with the invention since the inventive collimator provides improved construction and drive and the computing effort is considerably reduced. This method offers, in particular, high safety with regard to malfunction due to the forced coupling between the two drives.
The forced coupling of the drive of the rear parts of the collimator leaves and the actuator for the front parts can be realized by a transmission. To increase the space for the transmissions, the transmissions, optionally also the drives, can be disposed alternately at the top of one collimator leaf and at the bottom of the neighboring collimator leaf. This is particularly important if the collimator leaves must be very narrow, as is required for an exact reproduction of the shape of the tumor. A first embodiment of the drives provides that the actuator for the front parts is designed to align the front parts with respect to the radiation source when individual collimator leaves are adjusted, when all leaves are adjusted, or when some of the collimator leaves are adjusted. This permits movement of the overall collimator block or, through adjustment of the collimator block halves, to move them apart and thereby increase the collimator opening. This permits treatment of larger irradiation objects with a relatively small collimator without having to do without the inventive alignment of the front edges of the collimator leaves.
As drive, the rear part can have an associated collimator toothed rack into which a driving toothed wheel engages, wherein the collimator toothed rack associated with the rear part can also be embodied as teeth in a longitudinal edge of the rear part of the collimator leaves.
To provide good guidance of the collimator leaves, a rear section proximate the region of the gearing in the longitudinal edge of the rear part can be disposed in the collimator block at a displaced height such that a guiding element connected to the side of the collimator block above the gearing can engage in a guiding groove of the rear part or in the rear section. This produces secure guidance directly in the vicinity of the engagement region of the toothed wheel to provide exact play-free displacement of the collimator leaves. Clearly, additional guidance means can also be provided, e.g. a guidance for the edge of the rear part opposite to the gearing. In a particularly advantageous fashion, a guiding element is provided which is guided in a groove of the longitudinal edge of the rear part for securely retaining the collimator leaf in its position even if the neighboring collimator leaf assumes a substantially different position and is therefore no longer directly adjacent.
The driving means for adjusting the front part may be a front edge toothed rack which is hinged to the front part outside of its axis of rotation and into which a toothed gear engages to produce an adjustment path which differs from that of the rear part. The differing adjustment path permits corresponding adjustment of the front edge. Although this can be effected e.g. with separate drives, a forced mechanical coupling is preferred.
One embodiment of simple construction and reliable function provides that the collimator toothed rack and the front edge toothed rack are disposed on a longitudinal edge of the rear part and have different subdivisions to produce the differing adjustment paths, wherein a toothed wheel engages both toothed racks, wherein the subdivision difference lies within the tolerance limits of the gearing. With respect to a transmission disposed below a collimator, the subdivisioning of the front edge toothed rack is larger than that of the collimator toothed rack. With respect to a transmission disposed above a collimator, the subdivisioning of the front edge toothed rack must be smaller than that of the collimator toothed rack. This embodiment has, of course, only exemplary character and further possibilities are feasible, e.g. spindle drives with different pitches.
The driving toothed wheels or further toothed wheels which engage both toothed racks can be disposed in the collimator block. Alternatively, the further toothed wheels can be borne by the base frame. One toothed wheel can take over both functions or two separate toothed wheels can be provided.
In a further development, the driving toothed wheel engages in a driving toothed rack and is disposed in a displaceable collimator block or in two displaceable collimator block halves, and one further toothed wheel, which engages the collimator toothed rack and the front edge toothed rack, is disposed on a base frame. The collimator block as a whole or the collimator block halvesxe2x80x94one for each part of the collimator leavesxe2x80x94can be displaced on the base frame. The base frame can thereby be disposed between the collimator block and the gantry or the base frame can be the gantry itself. The driving toothed rack can be a separate toothed rack or a continuation of the collimator toothed rack relative to a shorter front edge toothed rack. This embodiment has the advantage that even when displacing the collimator block or the collimator halves, any setting of the front edges of the collimator leaves with respect to the radiation source, once adjusted, is maintained so that the front edges are always aligned parallel to the radiation even if the collimator block or the collimator block halves are adjusted. This is effected by the further toothed wheel which engages both toothed racks thereby ensuring the relative orientation and positioning of both toothed racks throughout the entire travel region to considerably increase the possible adjustment region and the collimator opening.
Of course, there are many further possible types of control and forced mechanical couplings. The drive for the rear parts can e.g. be connected to a link drive for the adjustment of the front parts. These link drives can have different designs. A connecting link guide can be directly connected to the bearing of the driving toothed wheel and a slider of the connecting link guide can cooperate with the front part. The connecting link guides can also be directly connected to a base frame and displaceable collimator block halvesxe2x80x94one for each part of the collimator leavesxe2x80x94are directly connected to the bearings of the driving toothed wheels. A concrete embodiment of a connecting link guide provides that a slider is mounted to a cable control which is guided to the front part and is mounted with one end above, and with the other end below the imaginary axis of rotation of the front part. A further possible embodiment consists in that the slider is mounted at a rear end of a two-armed lever wherein the axis of rotation of the lever is disposed on the rear part and its front end engages in the rear region of the front part to effect adjustment.
Preferably, a guidance is provided on at least one, preferably both longitudinal edges of each rear part which can be designed e.g. such that a groove is formed on the longitudinal edge in which a guiding element of the collimator block slides. Further possible guidance means are feasible to ensure that a collimator leaf is securely guided even when the neighboring collimator leaf is displaced to such an extent that the collimator leaf is exposed.
The collimator leaves can serve as compensating means for generating different radiation intensities by temporarily introducing them into the collimator opening during irradiation. This reduces the need for separate compensating means without excluding use thereof along with the inventive collimator.